1. DIAGNÓSTICO SITUACIONAL
1.6. Desarrollo de variables e indicadores
In nearly all cases, both S and R scores are assumed to be the same for bottom trawls and scallop dredges.5 This assumption seems reasonable since the disturbance caused by both gears is similar: aside from the trawl doors, both gears cause a scraping and smoothing of bottom features and a re-suspension of fine sediments. These effects are primarily limited to the sediment surface. While it is acknowledged that scallop gear may skim over the seabed somewhat, the features assessed, particularly the biological features, have a higher relief off the seafloor and thus are expected to be contacted by the gear. Furthermore, the scallop dredge impacts literature does not provide much support for a difference in S/R coding between gear types. In particular, for trawl gear matrix evaluations, the most useful types of studies were those that estimated
reductions in features following a single or multiple passes of experimentally fished gear. However, fewer scallop dredge impact studies were designed in this way, and those that did consider single pass impacts did so for geological features only. The studies that considered scallop dredge impacts to biological features were often
comparative examinations of unfished areas vs. areas fished by both dredges and trawls. In these instances, it is difficult to make inferences about the impacts of scallop dredges alone.
Table 24 shows scallop dredge gear S/R values, grouped by substrate and then by feature. Scores are the same for high and low energy unless otherwise noted. Table 25 summarizes the justifications for susceptibility scores for scallop dredge gear. Recovery
5 Despite the close similarities in the matrices, in terms of model outputs, the resulting adverse effects estimated for the two gear types will vary based on differences in gear dimensions, number of tows, and fishing locations.
scores for all gear types are combined into two tables at the conclusion of the matrix results section (Table 31 – geological, Table 32 - biological).
Table 24 – Scallop dredge matrices. Susceptibility (S) values are coded as follows: 0: 0-10%; 1: >10-25%; 2: >25-50%; 3: >50%. Recovery (R) values are coded as follows: 0: <1 year; 1: 1-2 years; 2: 2-5 years; 3: >5 years. The literature column indicates those studies identified during the literature review as corresponding to that combination of gear, feature, energy, and substrate. The studies referenced here were intended to be inclusive, so any particular study may or may not have directly informed the S or R score. Any literature used to estimate scores is referenced in Table 25 (Scallop dredge S), Table 31 (Geo R), and Table 32 (Bio R).
Gear: Scallop Substrate: Mud Feature name and class –
G (Geological) or B (Biological)
Gear effects Literature high Literature low S R
Biogenic burrows (G) filling, crushing none none 2 0
Biogenic depressions (G) filling 11 11 2 0
Sediments,
surface/subsurface (G) resuspension, compression, geochem, sorting, mixing 42, 236, 256, 391 none 2 0 Amphipods, tube-
dwelling (B) – see note crushing 228, 359 217 1 0
Anemones, cerianthid
burrowing (B) breaking, crushing, dislodging, displacing 228 217 2 2 Corals, sea pens (B) breaking, crushing, dislodging,
displacing 228 none energy 2 (low
only)
2 (low energy only) Hydroids (B) breaking, crushing, dislodging,
displacing 11, 228 11 1 1
Mollusks, epifaunal bivalve, Modiolus
modiolus (B)
breaking, crushing, dislodging,
displacing 42, 43, 256 203, 217 1 3
Substrate: Sand Feature name and class –
G (Geological) or B (Biological)
Gear effects Literature high Literature low S R
Bedforms (G) smoothing 11, 225, 236, 359 n/a 2 (high energy only)
0 (high energy only)
Biogenic burrows (G) filling, crushing 225 none 2 0
Biogenic depressions (G) filling 11, 225 ,359 11, 359 2 0 Sediments,
surface/subsurface (G) resuspension, compression, geochem, sorting/mixing 42, 119, 225, 236, 256, 352, 359, 391 none 2 0 Shell deposits (G) displacing, burying, crushing 11, 225, 352 11 1 1
(high), 2 (low) Amphipods, tube-
dwelling (B) – see note crushing 225, 228, 359 217 1 0 Anemones, cerianthid
burrowing (B) breaking, crushing, dislodging, displacing 70, 71, 228, 352 217 2 2 Ascidians (B) breaking, crushing, dislodging,
displacing 11, 352 203 2 1
Corals, sea pens (B) breaking, crushing, dislodging,
displacing 228 none energy 2 (low
only)
2 (low energy only) Hydroids (B) breaking, crushing, dislodging,
displacing 11, 69, 70, 71, 225, 228, 352 11 1 1 Mollusks, epifaunal
bivalve, Modiolus
modiolus (B)
breaking, crushing, dislodging,
Mollusks, epifaunal bivalve, Placopecten magellanicus (B) – see note breaking, crushing 42, 43, 69, 70, 71, 158, 352 203, 217 2 2 Polychaetes, Filograna
implexa (B) breaking, crushing, dislodging, displacing 11, 69, 70, 71, 158, 352 11, 217 2 2
Sponges (B) breaking, crushing, dislodging,
displacing 11, 70, 71, 225, 228, 352 203 2 2
Substrate: Granule-pebble Feature name and class –
G (Geological) or B (Biological)
Gear effects Literature high Literature low S R
Granule-pebble,
pavement (G) burial, mixing, homogenization none 1 (high energy only)
0 (high energy only) Granule-pebble,
scattered, in sand (G) burial, mixing 11, 43, 225, 352 11 1 (high), 0 2 (low) Shell deposits (G) burying, crushing, displacing 11, 225, 352 11 1 1
(high), 2 (low) Anemones, actinarian (B) breaking, crushing, dislodging,
displacing 11, 70, 71, 203, 225, 228, 352 none 2 2 Anemones, cerianthid
burrowing (B) breaking, crushing, dislodging, displacing 70, 71, 228, 352, 404 217 2 2 Ascidians (B) breaking, crushing, dislodging,
displacing 352 203 2 1
Brachiopods (B) breaking, crushing, dislodging,
displacing none none 2 2
Bryozoans (B) breaking, crushing, dislodging,
displacing 11, 69, 70, 71, 225, 228, 352, 404 11 1 1 Hydroids (B) breaking, crushing, dislodging,
displacing 11, 69, 70, 71, 225, 228, 352, 404 11 1 1 Macroalgae (B) breaking, dislodging none n/a 1 (high
energy only) 1 (high energy only) Mollusks, epifaunal bivalve, Modiolus modiolus (B)
breaking, crushing, dislodging,
displacing 43, 69, 70, 71, 158, 352, 404 203, 217 2 3 Mollusks, epifaunal bivalve, Placopecten magellanicus (B) – see note breaking, crushing 43, 69, 70, 71, 158, 352, 404 203, 217 2 2 Polychaetes, Filograna
implexa (B) breaking, crushing, dislodging, displacing 11, 69, 70, 71, 158, 352, 404 11, 217 2 2
Polychaetes, other tube-
dwelling (B) – see note crushing, dislodging 11, 69, 70, 71, 158, 352, 404 11, 217 2 1 Sponges (B) breaking, dislodging,
displacing 11, 70, 71, 225, 228, 352, 404 11, 203 2 2
Substrate: Cobble Feature name and class –
G (Geological) or B (Biological)
Gear effects Literature high Literature low S R
Cobble, pavement (G) burial, mixing,
homogenization none n/a 1 (high energy only)
0 (high energy only) Cobble, piled (G) smoothing, displacement none none 3 3 Cobble, scattered in sand
(G) burial, mixing, displacement 11, 43, 352 11 1 0
Anemones, actinarian (B) breaking, crushing, dislodging,
displacing 11, 70, 71, 228, 352 none 2 2
Ascidians (B) breaking, crushing, dislodging,
Brachiopods (B) breaking, crushing, dislodging,
displacing none none 2 2
Bryozoans (B) breaking, crushing, dislodging,
displacing 11, 69, 70, 71, 228, 352, 404 11 1 1 Hydroids (B) breaking, crushing, dislodging,
displacing 11, 69, 70, 71, 228, 352, 404 11 1 1 Macroalgae (B) breaking, dislodging none n/a 1 (high
energy only) 1 (high energy only) Mollusks, epifaunal bivalve, Modiolus modiolus (B)
breaking, crushing, dislodging,
displacing 43, 69, 70, 71, 158, 352, 404 217 2 3 Mollusks, epifaunal bivalve, Placopecten magellanicus (B) – see note breaking, crushing 43, 69, 70, 71, 158, 352, 404 217 2 2 Polychaetes, Filograna
implexa (B) breaking, crushing, dislodging, displacing 11, 69, 70, 71, 158, 352, 404 11, 217 2 2
Polychaetes, other tube-
dwelling (B) – see note crushing, dislodging 11, 69, 70, 71, 158, 352, 404 11, 217 2 1 Sponges (B) breaking, dislodging,
displacing 11, 70, 71, 228, 352, 404 11 2 2
Substrate: Boulder Feature name and class –
G (Geological) or B (Biological)
Gear effects Literature high Literature low S R
Boulder, piled (G) displacement none none 2 3
Boulder, scattered, in
sand (G) displacement 11, 43, 352 11 0 0
Anemones, actinarian (B) breaking, crushing, dislodging,
displacing 11, 352 none 2 2
Ascidians (B) breaking, crushing, dislodging,
displacing 11, 352 11 2 1
Brachiopods (B) breaking, crushing, dislodging,
displacing none none 2 2
Bryozoans (B) breaking, crushing, dislodging,
displacing 11, 352 11 1 1
Hydroids (B) breaking, crushing, dislodging,
displacing 11, 352 11 1 1
Macroalgae (B) breaking, dislodging none n/a 1 (high energy only) 1 (high energy only) Mollusks, epifaunal bivalve, Modiolus modiolus (B)
breaking, crushing, dislodging,
displacing 43, 352 217 2 3
Polychaetes, Filograna
implexa (B) breaking, crushing, dislodging, displacing 11, 352 11, 217 2 2
Polychaetes, other tube-
dwelling (B) – see note crushing, dislodging 11, 352 11, 217 2 1 Sponges (B) breaking, dislodging,
displacing 11, 352 11, 217 2 2
Note: Only references 217 and 225 are specific to tube-dwelling amphipods, the rest are derived from entries in database coded as prey/amphipods. Similarly, references for epifaunal bivalves/ scallops and other tube-dwelling polychaetes are based on database entries for epifaunal bivalves/mussels and polychaetes/F. implexa.
Table 25 – Scallop dredge susceptibility summary for structural features.
Feature Substrates
evaluated Score Notes
Amphipods, tube-dwelling Mud, sand 1 See trawls Anemones, actinarian Granule-
Feature Substrates
evaluated Score Notes
cobble, boulder Anemones, cerianthid
burrowing Mud, sand, granule- pebble 2 See trawls Ascidians Sand, granule- pebble, cobble, boulder
2 Molgula arenata removed from sand in linear patterns by
scallop dredges on Stellwagen Bank (11), degree of impact assumed to be same as trawls
Bedforms Sand 2 Multiple tows reduced frequency of sand waves in treatment areas compared to control areas (359), no information for single tows.
Biogenic burrows Mud, sand 2 Multiple tows reduced frequency of amphipod tube mats in treatment areas compared to control areas (359), no information for single tows.
Biogenic depressions Mud, sand 2 Multiple tows reduced frequency of biogenic depressions in treatment areas compared to control areas (359), no information for single tows.
Boulder, piled Boulder 2 No information, see trawls.
Boulder, scattered in sand Boulder 0 Single tows plowed boulders (43), but probability of burial is assumed to be low (see trawls).
Brachiopods Granule- pebble, cobble, boulder 2 See trawls Bryozoans Granule- pebble, cobble, boulder 1 See trawls
Cobble, pavement Cobble 1 Single tows dislodged cobbles (43) Cobble, piled Cobble 3
Cobble, scattered in sand Cobble 1 See trawls Corals, sea pens Mud, sand 2 See trawls Granule-pebble, pavement Granule-
pebble 1 Granule pebble, scattered
in sand Granule-pebble 1 Single tows overturned and buried gravel fragments (43) Hydroids Mud, sand,
granule- pebble, cobble, boulder 1 See trawls Macroalgae Granule- pebble, cobble, boulder 1 See trawls Mollusks, epifaunal
bivalve, Modiolus modiolus Mud, sand Granule-
1 2
Feature Substrates
evaluated Score Notes
pebble, cobble, boulder Mollusks, epifaunal bivalve, Placopecten magellanicus Sand, granule- pebble, cobble
2 Scallop dredge efficiency estimated to be 54% per tow (Gedamke et al. 2005), approximately 30% of scallops slightly buried after passage of 8 m dredge (42). Even if removal rates per tow are high (>50%), shucked shells returned to bottom still provide habitat value, so loss of functional value was assumed to be 25-50%.
Polychaetes, Filograna
implexa Sand, granule- pebble, cobble, boulder
2 See trawls
Polychaetes, other tube-
dwelling Granule-pebble, cobble, boulder
2 See trawls
Sediments, surface and
subsurface Mud, sand 2 Single tow lowered mud sediment surface 2 cm, mixed finer sediment to 5-9 cm, increasing mean grain size in upper 5 cm (236). Skids left furrows 2 cm deep in mixed mud/sand bottom, depression from tow bar, marks made by rings in chain belly of dredge (42, 43). Multiple tows in mud/muddy sand caused loss of fine sediments and reduced food value in top few cm (391). In sand, single tows re-suspended sand (43), multiple tows re- worked top 2-6 cm of sediments (359). Effects expected to be especially consequential in mud due to presence of biogenic matrix and because mud is more easily re-suspended by turbulence than sand (see trawls).
Shell deposits Sand, granule- pebble
1 Individual dredge tows dispersed shell fragments in troughs between sand waves (11), degree of impact assumed to be same as trawls. Sponges Sand, granule- pebble, cobble, boulder
2 Significantly more sponges at shallow sites undisturbed by trawls and scallop dredges on Georges Bank two years after area was closed, but not at deeper sites (404); for before/after impact experiments, see trawls.